Have you ever felt like making boozy eggnog is a total crapshoot? Half the time you get a smooth and delicious cocktail, and half the time it's a lumpy, curdled mess. What gives?

Turns out there's a formula you ought to be following. Here's the scientific secret that will get you a silky nog every time.

It's Friday afternoon, you've made it through the long week, and it's time for Happy Hour, Gizmodo's weekly booze column. A cocktail shaker full of innovation, science, and alcohol. Thirsty, Santa?

Breaking Down the Nog

Whether you're making it from scratch or buying it in a carton, the main ingredient is almost always milk. Eggs (especially yolks) are in there, too, as is sugar, cream, and probably a dash of nutmeg and other spices. It's the milk, however, that is the clotty culprit.

Milk is roughly 87 percent water. The other 13 percent is composed of fats, sugars, and other compounds, including proteins—and the proteins are what make milk so interesting in a mixture. Eighty percent of the protein in milk is casein (the other 20 percent is whey). Casein molecules float around in tiny clusters called micelles. These micelles are not attracted to each other under normal milky circumstances—but eggnog is not a normal circumstance.

Upset the pH Balance

Fresh milk typically has a pH of around 6.7. (Water is 7, so milk is just slightly more acidic.) However, once milk falls to about 6.5, the casein micelles try to cling together to survive the harsher, more acidic environment. When these proteins bind together, you get the big unappetizing curds. So what causes the pH to drop? Blame it on the booze.

Pure alcohol (ethanol) is almost perfectly neutral (pH 7), which wouldn't cause any reaction in the milk. But spirits aren't pure alcohol. Most spirits are only 40 percent pure alcohol, and the rest of the solution is quite acidic. The stuff you're most likely to add to eggnog—brandy, bourbon, or rum—can have a pH all the way down in the 3's or 4's. So a shot of hooch is more than enough to tip the scales on a to a cup of milky nog. In fact, even beer is acidic enough to curdle Irish cream, and drinkers of a certain "bomb" drink can tell you. However, there is one clot-fighting hero in this game.

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Get Friendly with Fat

Lipids—the emulsified fats in milk—can slow the coagulation of the casein proteins. Additionally, the fattier the milk, the lower the concentration of casein in it will be. Skim milk starts clotting the second it hits booze. Whole milk is typically 4-percent fat, and while it works better than 2-percent or 1-percent, it's generally not fatty enough for a strong drink. This is why drinks like White Russians calls for at least half-and-half (typically 10-18 percent fat), or light cream (18-30 percent), or a whipping cream (30-36 percent), or even a heavy cream—that's 36 percent fat and higher.

Are these low-cal, heart-healthy choices? No. But will they ensure a silky, chunk-free nog experience? Probably. If you know you're going to be adding alcohol to your nog, do not buy the light stuff at the store. If you're making it at home, don't skimp on the cream. That said, if you're afraid your heart is going to instantaneously explode, you may be able to find a "cooking cream" that is lower in fat. These are manufactured with additives to help it resist curdling and separating. But we can't vouch for the flavor.

The Order of Operations

Any good eggnog recipe (like this one from Martha Stewart) has two parts—preparing the dairy and then adding the alcohol. Your instinct might be to pour booze into the glass first, to make sure you're not taking the world's largest shot, and then add eggnog until the flavors are right. But a difference in concentrations of pH between the two solutions means that is more likely to result in curdling. The alcohol is an comparatively very acidic solution, so when you're first adding a milk product to it, you are exposing the milk (and the casein micelles) to a very acidic environment, and they will immediately start binding together.

The right way to do it is by pouring the nog first, and then slowly pouring in the booze while stirring. Finish up all the whisking of the cream, sugar, and eggs, and at the end, add in in the bourbon (and/or rye, rum, brandy, etc). Add acid: It's a law of the lab. In this case, it uses a high volume of fat content to give you the highest chance of clot-free success.

Do Curdles Make You Sick?

There are a lot of people in the internetz who believe that if the milk in your eggnog or cocktail curdles, it will make you projectile vomit the Star Spangled Banner. This is wrong. Here's proof. Remember what curdles milk? Acid. Do you know what's full of acid? Your stomach. Every time you drink milk, it curdles, you just can't see it. If curds made you sick you wouldn't be able to eat cottage cheese, would you?

Milk makes you sick is when it's spoiled, which also happens to cause curdling. As milk ages, its pH drops (i.e., it becomes more acidic), which is why it tastes sour. Once it reaches a certain point (around 5.5 pH), it curdles in its own acid. That's a good reason to use the freshest dairy products you can get for your cocktail—beyond being tasty, they'll be more resistant to curdling.

Can the Booze Stop Salmonella?

Yes and no. There was a very informal study done a few years ago by Microbiologists Vince Fischetti and Raymond Schuch at Rockefeller University. They found that the alcohol in the eggnog was indeed capable of killing salmonella—but it took three weeks to do it. After just one day, there was hardly any reduction in the bacteria at all.

Since the holidays are upon us and most of you don't have three weeks to kill, do not rely on booze to sterilize your nog. Simply cooking the egg yolks in a homemade batch, as instructed in Martha's recipe above, is a good precaution.

Cheers!

There are a few different ways to go about making eggnog, but for most any recipe, the same science should apply. Do you have a magical family eggnog formula? We hope you'll share it in the comments below. If you don't, we highly recommend this recipe from our friend Alton Brown. It's pretty much unbeatable.

From us at Gizmodo, drink responsibly, don't drive, and have a lovely and safe holiday season. Cheers, and we'll see you next week for more Happy Hour.